I finally started PVP pet battles last night. After getting whupped 4x in a row, I thought I would try a long haul approach and started with my S/S roach, cast Apocalypse and lasted through until it detonated. To my dismay, it only killed the front line pet, and the 2 in the back line took no damage at all. Do the same rules for abilities not apply in PVP battle? If not, is there somewhere I can learn what the differences are?

Apocalypse only kills active pets. "Calls down a meteor, which will fall 15 rounds from now, instantly killing all active pets."Back line pets are not considered active until you bring them into the fight, at least this is my understanding.

Correct. This is a team I play all the time and Apocalypse only kills the active pet. Use the beetle or roach cast Apocalypse swap out soon put the beatle or raoch back in right when Apocalypse will hit.

It's one pet only, or every PvP team would just consist of cockroaches. Also note that the Apocalypse doesn't necessarily kill any active pet. Magic type pets with more than 40% health will survive thanks to the family ability, undead pets will get their bonus round(s), mechanical pets will self-repair, pets with Survival will retain one hit point, etc.